The firing order is sequential, i.e No4 on engine 1, No2 on engine 2, No 6 on engine 3, 3 on 4, 5 on 5, 1 on 1, 4 on 2 etc etc. Because the cranks rotate anti clockwise looking at the timing end, the firing order of each block is reversed to the norm. It is very smooth and starts (normally) instantly.